Such was the lure - and challenge - of Grudge Match, the comedy that pairs Sylvester Stallone and Robert De Niro as aging boxing greats who meet for one final battle in the ring. Kim Basinger co-stars in the film, due out Dec. 25.

While Grudge won't sport the names Rocky Balboa or Jake "the Raging Bull'' LaMotta - the characters who would help define the careers of Stallone and De Niro more than 30 years ago - director Peter Segal (Anger Management, 50 First Dates) found a way to infuse his film with the icons of the ring.

"We found this amazing footage of Sly training for Rocky and Bob training for Raging Bull, and put parts of both in the movie," Segal says. Bull director Martin Scorsese even shot some 16mm footage of De Niro training with former middleweight champion LaMotta, and gave portions of the home movies to Segal. "Seeing them train for these iconic roles was watching film history."

Making some history of his own became Segal's mission after he and writers settled on the "Rocky vs. Raging Bull" theme. Problem was, "once we settled on that, we could only do it with Sly and Bob."

While De Niro, 70, had poked fun at his legend with Analyze This, the 1999 comedy that lampooned his Godfather mystique, "Sly had never really winked at his iconic character. And he was nervous. But once he got into it, he lost all the nervousness."

Stallone, 67, says, nerves aside, that he remains drawn to stories of the sweet science.

"I'm comfortable" in the ring, he says in an e-mail from Bulgaria, where he is filming The Expendables 3. "I love the metaphor for what a struggle life can be."

And he considers Bull "perhaps the most unapologetic biography ever made."

So who would win, Rocky or Raging Bull? Debate got so heated among crew and extras that Segal shot three finales to Grudge: In one, Stallone's character "Razor" wins; De Niro's "The Kid" wins in another; and both die in the third version.

"We had more than 500 extras for the big fight, and didn't want (the outcome) getting out," Segal says.

And Stallone isn't providing spoilers, though he does praise his opponent.

"Robert De Niro is a great actor," Stallone writes. "He is very reserved and very intelligent - and he takes a good punch."